1. Field of the Invention
The present invention relates to error-control techniques for mobile cellular data communication systems, and more specifically, to trellis coding of 16 Star Quadrature Amplitude Modulation (QAM).
2. Brief Description of the Related Art
In recent years, much of the research and development in the communications industry has been concentrated in the area of wireless data transmission. As is well known in the art, wireless data transmission typically involves transmission of data with a carrier frequency. The carrier frequency is modulated by the data so that a frequency bandwidth is occupied by the transmitted signal. The growing demand for access to mobile data and communication services has placed a significant strain on the available bandwidth. Moreover, there is an ever increasing demand for increased data communication rates for the purpose of decreasing the data transmission time. An increase of the rate of the data used to modulate the carrier frequency typically results in an increased bandwidth requirement, placing a further strain upon the available bandwidth for transmission of wireless signals.
In an effort to increase the data rates without sacrificing the available bandwidth, a number of modulation schemes together with sophisticated coding techniques have been developed. For example, Quadrature Amplitude Modulation (QAM) employs both amplitude and phase modulation in order to encode more data within a given frequency bandwidth. Another modulation technique involves Multiple Phase Shift Keying (MPSK) to increase data capacity within a given bandwidth. These high level modulation schemes are very sensitive to channel impairments. That is, the information encoded by means of such techniques is often lost during transmission due to noise, Rayleigh fading and other factors which are introduced over the communication medium.
In order to compensate for the increased sensitivity of these high level modulation schemes, various forward error correction coding techniques are employed. One widely accepted error coding technique is trellis coded modulation. Trellis coded modulation is highly desirable since it combines the operations of modulation and error coding to provide effective error control coding without sacrificing power and bandwidth efficiency. Trellis codes have been developed for many of the high level, high rate modulation schemes, including well-known 8-PSK modulation and Square 16 QAM modulation.